Sintered uranium dioxide pellets are loaded in cladding tubes to make fuel rods for the fuel rod assemblies forming the cores of nuclear reactors.
These pellets, and also other kinds of ceramic shapes, are produced by compressing the material in powder form, to form green compacts which are then sintered in high-temperature continuous furnaces.
A common type of continuous furnace is horizontal, has mutually opposite open ends, and a horizontal roller conveyor that extends through the furnace. The green compacts are loaded on boats in the form of stacked layers of compacts, and the boats are pushed over the roller bed and through the furnace, with one boat pushing the other so that the power means required to move the boat can be located outside of the furnace adjacent to the furnace's entrance end. To separate the layers, each layer is loaded on a carrier sheet so that when the boat is fully loaded each layer is separated by a carrier sheet.
The green compacts can be produced by automatic machinery, and if the carrier sheets are completely flat, they can be loaded by the compacts being pushed onto the sheets in automatic manner, eliminating any need for hand-loading.
As the boats are pushed through the furnace with each preceding boat being pushed by a succeeding boat, the boats receive a certain amount of jostling, this requiring that the sheets and compacts be restrained horizontally so that the sheets and compacts are carried safely through the furnace.
Such restraint has been provided by a box on the boat and made of thin sheet metal of sufficient height to accommodate the stacked layers of compacts with each layer loaded on a carrier sheet, the boat having four upstanding corner posts embraced by the corners of the box. The walls of the box are extensively perforated in an effort to assure that the furnace atmosphere, which must be inert with respect to the compacts at their sintering temperature, has complete access to the compacts.
The above prior art arrangement, incidentally, disclosed by German Pat. No. 1,583,749, is not completely satisfactory. Even though the box is made of the most heat-resistant metal that is commercially practical for use, the contour of the box is such that it is inherently subjected to warpage, keeping in mind that during the progress through the continuous furnace which normally includes both heating and cooling zones, the box is subjected to severe thermal stock. In addition, in spite of the perforations through the box walls, circulation of the furnace atmosphere through the stacked rows of sintered compacts, is substantially impeded, possibly resulting in changes in the chemical composition of the compact material.
The described prior art arrangement does have the advantage that the sheets can be made flat so that the automatic machinery producing the compacts can push the compacts onto the sheets so that manual loading of the sheets with the compacts, is avoided.
The desired disadvantages are particularly objectionable in connection with the sintering of uranium dioxide pellets used for the loading of nuclear reactor fuel rod cladding tubes, because of the necessity for holding the pellets exactly to their desired composition. In addition, the operation of any nuclear installation is extremely expensive and it is desirable to effect every possible economy, making the necessary replacement of the warped boxes made of expensive heat-resistant metal particularly undesirable. At the same time, considering the expense of uranium, any boat construction that might be considered as a substitute, should meet the requirements concerning automatic loading of the carrier sheets and free access of the furnace atmosphere to the stacked pellets, together with safe carriage through the continuous furnace.